This occurs when gclient compares the cached deps in .gclient_entries
to the new DEPS and tries to construct an SCM for any cached entries
which no longer exist. In this situation gclient doesn't account for
the case where previous entries may have been ignored by setting their
url to None via the custom_deps section in .gclient.

Contributed by Jay Soffian.

Review URL: http://codereview.chromium.org/3769002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@62540 0039d316-1c4b-4281-b951-d872f2087c98